Field of the Invention
The present invention relates to optical fibers.
Technical Background
In optical communication systems, optical-fiber transmission paths are used as optical transmission paths that transmit signal light. In signal light transmission (i.e., digital coherent transmission in particular), it is desirable that the OSNR (optical signal-to-noise ratio) be improved. In order to achieve this, it is required to reduce attenuation and nonlinearity of an optical fiber. In order to reduce nonlinearity of an optical fiber, it is effective to increase the effective area of the optical fiber. For example, an optical fiber with an effective area increased to 110 μm2 or larger is known. However, in an optical fiber with an increased effective area, the confinement of propagation light (fundamental-mode light) to a core weakens and a micro-bend loss tends to increase, ultimately causing the OSNR to become lower. A micro-bend loss is a phenomenon in which a transmission loss increases due to a leakage of light caused when a fundamental mode propagating through the core is coupled with a cladding mode due to random micro-bending occurring as a result of stress applied to the optical fiber from a side.
Pierre Sillard, et al., “Micro-Bend Losses of Trench-Assisted Single-Mode Fibers,” ECOC2010 We.8.F.3 (Non-Patent Literature 1) describes a micro-bend loss. According to Non Patent Literature 1, in a single-mode optical fiber having step-shaped refractive-index distribution, the micro-bend loss increases exponentially relative to the wavelength. In an optical fiber having trench-shaped refractive-index distribution, random micro-bending causes the fundamental mode to be coupled with a higher-order mode, which is a leakage mode, so that the micro-bend loss is relatively uniform with respect to the wavelength. Trench-shaped refractive-index distribution has a core, first cladding, second cladding, and third cladding in that order from the center. The refractive index of the core is larger than the refractive indices of the first cladding and the third cladding, and the refractive index of the second cladding is smaller than the refractive indices of the first cladding and the third cladding.
W-shaped refractive-index distribution is known as refractive-index distribution of an optical fiber in which the effective area thereof can be increased. W-shaped refractive-index distribution has a core, inner cladding that surrounds the core and has a refractive index smaller than the refractive index of the core, and outer cladding that surrounds the inner cladding and has a refractive index smaller than the refractive index of the core and larger than the refractive index of the inner cladding. The wavelength dependency (spectrum) of the micro-bend loss of an optical fiber having W-shaped refractive-index distribution is not described in Non Patent Literature 1.